MONOCOTYLEDONS. 



301 



March, when some four leaves were visible ; 45 per cent, was 

 added by the time it was in full flower at the end of June ; 

 and the remaining 35 per cent, before harvest. This 35 per 

 cent, is not, however, an addition to the weight of the plant 

 by the production of new food ; the greater part of it had 

 already been prepared and had been sent down into the roots 

 to be in store there until it was needed. When the flowering 

 is finished the food is transferred from the underground 

 parts of the plant to be finally stored in the seed, and neces- 

 sarily increases the weight of dry matter in the plant above 

 ground. 



The stem or " straw " of the Wheat plant is hardened by 

 the fibrous tissue surrounding the bundles, which form a 

 hollow cylinder, and additional strength is gained by the 

 presence of flinty matter (silica) in the epidermis. The stem 

 has alternate green and white lines, like the leaf, the green 

 lines bearing stomates. 



It is interesting to compare the strength of the " straw," 

 which has to bear the weight of the heavy ripe spike, with 

 that of other materials. Cut off ripe spikes and attach them 

 (tie with thread, or stick with sealing-wax) to wires of steel, 

 iron, copper, of about the same length and weight as the 

 Wheat- stalk. The latter is a fine example of strength com- 

 bined with lightness and economy of material. The "nodes" 

 more strictly, the swollen bases of the leaf -sheaths are of 

 great importance. The actual node forms a solid partition 

 across the otherwise hollow stem and thus gives it greater 

 rigidity ; the swollen base of the leaf-sheath, just above the 

 "node" itself, remains capable of growth, and, in the lower 

 parts of the plant especially, if the stem is laid horizontally 

 (e.g. by wind), the lower side of the "node" grows rapidly 

 and thus brings the shoot into the vertical position again; 

 the lower " nodes " also give out roots. 



Examine young ears and study the stages in the ripening 

 of the fruit ; the stigmas and stamens fall off after fertilisa- 

 tion, and the ovary grows into the grain. 



316. Oat (Avena sativa) is probably derived by cultiva- 

 tion from the Wild Oat-grass (A. fatua). It can be culti- 

 vated above the limits of height and of latitude at which the 

 cultivation of Wheat stops. In this country Wheat ceases 



